I was just looking at the Keset haSofer and came across an interesting halacha

ואין נוהגין כן אלא כותבין בנוצה ואפי׳ בקולמס של ברזל

This is to say that we may write with a metal kulmus * (pen for writing sifrei kodesh). I was under the impression that it is preferable not to base metal implements in the creation of holy things, as they can also be used for creating weapons**

What is his rationale for permitting an iron implement for writing sta"m and do others agree with him?

3 Answers
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The Rama writes that some say to use a reed and not a feather. The Taz explains, that by a Get there is a halacha that it should not be written using metal because it might engrave the words, and that would not disqualify the get. By sta"m - Sefer Torah, Tefillin and Mezuzahs- we want to avoid that as well, and that is why even a feather shouldn't be used, only a soft reed.

The Taz and Shach quote the Levush who says that it is not the minhag to avoid using feathers. The Keset HaSofer - you mentioned - takes it further, that we are not concerned about the scratching even with metal.

None of these sources write about the weapons aspect.

However, the Aruch HaShulchan does give two reasons why not to write with metal: 1 - it makes holes in the parchment, 2 - "The Torah increases life and metal shortens it".

P.S. Rav Wosner writes that one should not deviate from the minhag to use feathers unless there is a need.

I"m not really answering your question about the rational, other than to point out that the Aruch Hashulchan uses that rational, but I am trying to answer the part about others agreeing.

Sorry for reviving an old thread. The use of metal vs. other materials for writing is mostly based on practicality (writing properties and properties of the klaf), although there are those who prefer to use a feather specifically based on the concept that just like the klaf should be from an edible animal, so too should the kulmus.

Regarding the tzitzis portion of the question, I don't have the source off hand, but I've seen/heard that tzitzis should be cut with one's teeth. When I had my tallis cleaned, the proprietor of the store indicated that the frayed ends should be cut with ceramic scissors, because it supposedly makes it less prone to fraying vs. a cut with metallic scissors. FWIW, I did try trimming with my teeth and fraying was minimal afterwards.